The Role of Gas in the Merging of Massive Black Holes in Galactic Nuclei (original) (raw)
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Astrophysical Journal, 2005
Using high-resolution SPH numerical simulations, we investigate the effects of gas on the in-spiral and merger of a massive black hole binary. This study is motivated by the very massive nuclear gas disks observed in the central regions of merging galaxies. Here we present results that expand on the treatment in a previous work by studying models in which the gas is in a disk. We run a variety of models, ranging from simulations with a relatively smooth gas disk to cases in which the gas has a more clumpy spatial distribution. We also vary the inclination angle between the plane of the binary and the plane of the disk, and the mass ratio between the MBHs and the gaseous disk. We find that, as in our previous work, in the early evolution of the system the binary separation diminishes mainly due to dynamical friction exerted by the background gas, and in the later stages the gaseous medium responds by forming an ellipsoidal density enhancement whose axis lags behind the binary axis. This offset produces a gravitational torque on the binary that causes continuing loss of angular momentum and is able to reduce the separation to distances at which gravitational radiation is efficient. The main difference is that between these two regimes we now find a new transition regime that was not apparent in our previous paper, in which the evolution is temporarily slowed down when neither of these mechanisms is fully effective. In the variety of simulations that we perform, we find that the coalescence timescale for the MBH binary varies between 5 ; 10 6 and 2:5 ; 10 7 yr for typical ULIRGs. For MBHs that satisfy the observed ''mc '' relation, our simulations suggest that in a merger of galaxies that have at least 1% of their total mass in gas, the MBHs will coalesce soon after the galaxies merge.
Astrophysical Journal, 2003
Using high-resolution smoothed particle hydrodynamics numerical simulations, we investigate the effects of gas on the in-spiral and merger of a massive black hole binary. This study is motivated by both observational and theoretical work that indicate the presence of large amounts of gas in the central regions of merging galaxies. N-body simulations have shown that the coalescence of a massive black hole binary eventually stalls in a stellar background. However, our simulations suggest that the massive black hole binary will finally merge if it is embedded in a gaseous background. Here we present results in which the gas is assumed to be initially spherical with a relatively smooth distribution. In the early evolution of the binary, the separation decreases as a result of the gravitational drag exerted by the background gas. In the later stages, when the binary dominates the gravitational potential in its vicinity, the medium responds by forming an ellipsoidal density enhancement whose axis lags behind the binary axis, and this offset produces a torque on the binary that causes continuing loss of angular momentum and is able to reduce the binary separation to distances where gravitational radiation is efficient. Assuming typical parameters from observations of ultraluminous infrared galaxies, we predict that a black hole binary will merge within 10 7 yr; therefore, these results imply that in a merger of gas-rich galaxies, any massive central black holes will coalesce soon after the galaxies merge. Our work thus supports scenarios of massive black hole evolution and growth in which hierarchical merging plays an important role. The final coalescence of the black holes leads to gravitational radiation emission that would be detectable out to high redshift by LISA. We show that similar physical effects, which we simulate with higher resolution than in previous work, can also be important for the formation of close binary stars.
The Astrophysical Journal, 2005
We analyze the effect of dissipation on the orbital evolution of supermassive black holes (SMBHs) using highresolution self-consistent gasdynamical simulations of binary equal-and unequal-mass mergers of disk galaxies. The galaxy models are consistent with the ΛCDM paradigm of structure formation and the simulations include the effects of radiative cooling and star formation. We find that equal-mass mergers always lead to the formation of a close SMBH pair at the center of the remnant with separations limited solely by the adopted force resolution of ∼ 100 pc. Instead, the final SMBH separation in unequal-mass mergers depends sensitively on how the central structure of the merging galaxies is modified by dissipation. In the absence of dissipation, the satellite galaxy can be entirely disrupted before the merger is completed leaving its SMBH wandering at a distance too far from the center of the remnant for the formation of a close pair. In contrast, we show that gas cooling facilitates the pairing process by increasing the resilience of the companion galaxy to tidal disruption. Moreover, we demonstrate that merging disk galaxies constructed to obey the M BH -σ relation, move relative to it depending on whether they undergo a dissipational or collisionless merger, regardless of the mass ratio of the merging systems. Collisionless simulations reveal that remnants tend to move away from the mean relation highlighting the role of gas-poor mergers as a possible source of scatter. In dissipational mergers, the interplay between strong gas inflows associated with the formation of massive nuclear disks and the consumption of gas by star formation provides the necessary fuel to the SMBHs and allows the merger remnants to satisfy the relation.
Black hole fueling and coalescence in galaxy mergers
Astronomische Nachrichten, 2020
We study the gas accretion and orbital decay of super-massive black hole (SMBH) pairs in the nuclear region of galaxy merger remnants, where a large amount of gas is expected to accumulate. For that purpose, we perform numerical simulations with both smooth particle hydro-dynamics (SPH) (Gadget) and adaptive mesh refinement (AMR) (Ramses) techniques. We study the places of the highest SMBH gas accretion, finding that peaks of mass accretion rates are near pericenter passages. We also study the conditions for fast migration and coalescence of SMBHs in galactic nuclei, finding that, in the typical conditions of relatively wet mergers, the SMBHs will experience a fast shrinking, down to the scales where their final coalescence will be driven by the emission of gravitational waves, unless other processes strongly affect the distribution of the gas near the SMBH binary, such as active galactic nuclei (AGN) feedback.
Numerical Simulations of Merging Clusters of Galaxies
The Astrophysical Journal Supplement Series, 1997
We present results from three-dimensional numerical simulations of head-on mergers between two clusters of galaxies using a hybrid hydro/N-body code. In these simulations, the gaseous intracluster medium (ICM) is evolved as a massless Ñuid within a changing gravitational potential deÐned by the collisionless dark matter component. The ICM is represented by the equations of hydrodynamics which are solved by an Eulerian, Ðnite-di †erence method. The cluster dark matter component is represented by the N-body particle distribution. A series of simulations have been conducted in which we have systematically varied the cluster-subcluster mass ratio between 8 : 1 and 1 : 1. We Ðnd that cluster-subcluster mergers result in an elongation of both the cluster dark matter and gas distributions. The dark matter distribution is elongated parallel to the merger axis and accompanied by anisotropy in the dark matter velocity dispersion. Both the elongation and corresponding velocity anisotropy are sustained for more than 5 Gyr after the merger. The elongation of the gas distribution is also generally along the merger axis, although shocks and adiabatic compressions produce elongations perpendicular to the merger axis at various times during the merger. We also Ðnd a signiÐcant o †set between dark matter and gas centroids in the period following core passage. The gasdynamics is also severely a †ected by the cluster-subcluster merger. In these simulations, the subcluster enters the primary at supersonic speeds initiating bulk Ñows that can exceed 2000 km s~1. The width of the bulk Ñows are seen to range between several hundred kiloparsecs to nearly 1 Mpc. We believe the bulk Ñows can produce the bending of wide-angle tailed (WAT) radio sources. The most signiÐcant gasdynamics is seen to subside on timescales of 2 Gyr, although still signiÐcant dynamics is seen even after 5 Gyr. The merger-induced gasdynamics may also play a role in the formation of radio halo sources, and, consequently, the sustained nature of the gasdynamics may extend the lifetime of halos beyond the canonical synchrotron lifetime of the source. Substructure, shocks, and adiabatic cooling during the merger can result in a very complex temperature structure within the intracluster medium. As a result of these mergers, we Ðnd temperature inhomogeneities of several keV on linear scales of ¹0.5 Mpc. Finally, these simulations indicate that even relatively high mass-ratio mergers (e.g., 8 : 1) result in nonequilibrium conditions for an extended period of time. The period of time with the most signiÐcant dynamical evolution is within 2 Gyr after core passage. The nonequilibrium conditions have implications for cluster mass estimates. The observable consequences of cluster mergers and their inÑuence on cluster mass estimates are addressed in Roettiger, Burns, & Loken (1996).
Pairing and sinking of binary SMBHs in sub-pc resolution simulations of galaxy mergers
We study the formation of a supermassive black hole (SMBH) binary and the shrinking of the separation of the two holes to sub-pc scales starting from a realistic major merger between two gas-rich spiral galaxies with mass comparable to our Milky Way. The simulations, carried out with the Adaptive Mesh Refinement (AMR) code RAMSES, are capable of resolving separations as small as 0.1 pc. The collision of the two galaxies produces a gravo-turbulent rotating nuclear disk with mass (∼ 10 9 M ) and size (∼ 60pc) in excellent agreement with previous SPH simulations with particle splitting that used a similar setup ) but were limited to separations of a few parsecs. The AMR results confirm that the two black holes sink rapidly as a result of dynamical friction onto the gaseous background, reaching a separation of 1 pc in less than 10 7 yr. We show that the dynamical friction wake is well resolved by our model and we find good agreement with analytical predictions of the drag force as a function of the Mach number. Below 1 pc, black hole pairing slows down significantly, as the relative velocity between the sinking SMBH becomes highly subsonic and the mass contained within their orbit falls below the mass of the binary itself, rendering dynamical friction ineffective. In this final stage, the black holes have not opened a gap as the gaseous background is highly pressurized in the center. Non-axisymmetric gas torques do not arise to restart sinking in absence of efficient dynamical friction, at variance with previous calculations using idealized equilibrium nuclear disk models. We believe that the rather "hot" Equation-of-State we used to model the multiphase turbulent ISM in the nuclear region is playing an important role in preventing efficient SMBH sinking inside the central parsec. We conclude with a discussion of the way forward to address sinking in gaseous backgrounds at sub-pc scales approaching the gravitational wave regime.
The Astrophysical Journal, 2012
Galaxy centers are residing places for Super Massive Black Holes (SMBHs). Galaxy mergers bring SMBHs close together to form gravitationally bound binary systems which, if able to coalesce in less than a Hubble time, would be one of the most promising sources of gravitational waves for the Laser Interferometer Space Antenna (LISA). In spherical galaxy models, SMBH binaries stall at a separation of approximately one parsec, leading to the "final parsec problem" (FPP). On the other hand, it has been shown that merger-induced triaxiality of the remnant in equal-mass mergers is capable of supporting a constant supply of stars on so-called centrophilic orbits that interact with the binary and thus avoid the FPP. In this paper, using a set of direct N -body simulations of mergers of initially spherically symmetric galaxies with different mass ratios, we show that the merger-induced triaxiality is also able to drive unequal-mass SMBH binaries to coalescence. The binary hardening rates are high and depend only weakly on the mass ratios of SMBHs for a wide range of mass ratios q. There is, however, an abrupt transition in the hardening rates for mergers with mass ratios somewhere between q ∼ 0.05 and 0.1, resulting from the monotonic decrease of merger-induced triaxiality with mass ratio q, as the secondary galaxy becomes too small and light to significantly perturb the primary, i.e., the more massive one. The hardening rates are significantly higher for galaxies having steep cusps in comparison with those having shallow cups at centers. The evolution of the binary SMBH leads to relatively shallower inner slopes at the centers of the merger remnants. The stellar mass displaced by the SMBH binary on its way to coalescence is ∼ 1 − 5 times the combined mass of binary SMBHs. The coalescence time scales for SMBH binary with mass ∼ 10 6 M ⊙ are less than 1 Gyr and for those at the upper end of SMBH masses 10 9 M ⊙ are 1-2 Gyr for less eccentric binaries whereas less than 1 Gyr for highly eccentric binaries. SMBH binaries are thus expected to be promising sources of gravitational waves at low and high redshifts.
Swift Coalescence of Supermassive Black Holes in Cosmological Mergers of Massive Galaxies
The Astrophysical Journal, 2016
Supermassive black holes (SMBHs) are ubiquitous in galaxies with a sizable mass. It is expected that a pair of SMBHs originally in the nuclei of two merging galaxies would form a binary and eventually coalesce via a burst of gravitational waves. So far, theoretical models and simulations, focusing only on limited phases of the orbital decay of SMBHs under idealized conditions of the galaxy hosts, have been unable to directly predict the SMBH merger timescale from ab-initio galaxy formation theory. The predicted SMBH merger timescales are long, of order Gyrs, which could be problematic for future gravitational wave (GW) searches. Here, we present the first multi-scale ΛCDM cosmological simulation that follows the orbital decay of a pair of SMBHs in a merger of two typical massive galaxies atz 3, all the way to the final coalescence driven by GW emission. The two SMBHs, with masses~10 8 M , settle quickly in the nucleus of the merger remnant. The remnant is triaxial and extremely dense due to the dissipative nature of the merger and the intrinsic compactness of galaxies at high redshift. Such properties naturally allow a very efficient hardening of the SMBH binary. The SMBH merger occurs in only ∼10 Myr after the galactic cores have merged, which is two orders of magnitude smaller than the Hubble time.
FORMATION AND HARDENING OF SUPERMASSIVE BLACK HOLE BINARIES IN MINOR MERGERS OF DISK GALAXIES
The Astrophysical Journal, 2012
We model for the first time the complete orbital evolution of a pair of supermassive black holes (SMBHs) in a 1:10 galaxy merger of two disk-dominated gas-rich galaxies, from the stage prior to the formation of the binary up to the onset of gravitational wave (GW) emission when the binary separation has shrunk to 1 mpc. The high-resolution smoothed particle hydrodynamics (SPH) simulations used for the first phase of the evolution include star formation, accretion onto the SMBHs as well as feedback from supernovae explosions, and radiative heating from the SMBHs themselves. Using the direct N-body code φ-GPU, we evolve the system further without including the effect of gas, which in the mean time has been mostly consumed by star formation. We start at the time when the separation between two SMBHs is ∼700 pc and the two black holes are still embedded in their galaxy cusps. We use three million particles to study the formation and evolution of the SMBH binary until it becomes hard. After a hard binary is formed, we reduce (reselect) the particles to 1.15 million and follow the subsequent shrinking of the SMBH binary due to three-body encounters with the stars. We find approximately constant hardening rates and that the SMBH binary rapidly develops a high eccentricity. Similar hardening rates and eccentricity values were reported in earlier studies of SMBH binary evolution in the merging of dissipationless spherical galaxy models. The estimated coalescence time is ∼5.5 Gyr, significantly smaller than a Hubble time. We discuss why this timescale should be regarded as an upper limit. Since 1:10 mergers are among the most common interaction events for galaxies at all cosmic epochs, we argue that several SMBH binaries should be detected with currently planned space-borne GW interferometers, whose sensitivity will be especially high for SMBHs in the mass range considered here.
2008
Binary supermassive black holes form naturally in galaxy mergers, but their long-term evolution is uncertain. In spherical galaxies, N-body simulations show that binary evolution stalls at separations much too large for significant emission of gravitational waves (the “final parsec problem”). Here, we follow the long-term evolution of a massive binary in more realistic, triaxial and rotating galaxy models. We find that the binary does not stall. The binary hardening rates that we observe are sufficient to allow complete coalescence of binary SBHs in 10 Gyr or less, even in the absence of collisional loss-cone refilling or gas-dynamical torques, thus providing a potential solution to the final parsec problem. Subject headings: 1.